Abstract

From 2009 to 2011, the Canadian Prairies were subjected to exceptionally variable precipitation regimes, ranging between record drought and unprecedented flooding. Adjacent regions concurrently experienced droughts and floods, and individual areas transitioned rapidly from pluvial to drought conditions and vice versa. Such events had major impacts; for example, damages from floods in the Assiniboine River Basin (ARB) have exceeded $1 billion, and forest fires ravaged the town of Slave Lake, Alberta. This study first characterizes, and then assesses, these devastating natural hazards in terms of their physical processes (across multiple spatial and temporal scales) related to both the spatially contrasting precipitation states and rapid temporal transitions between these states. Subtle differences in large-scale atmospheric flow had marked impacts on precipitation. Primary factors controlling the distribution and amount of precipitation included the location and persistence of key surface and upper-air features, as well as their interaction. Additionally, multiple events—rather than individual extremes—were responsible for the flooding over the Saskatchewan River Basin and the ARB. Very heavy rainfall events (≥25 mm d−1) accounted for up to 55 % of warm season rain at some locations, and the frequency of heavy rainfall events was critical for determining whether a region experienced drought or pluvial conditions. This study has increased our knowledge of the characteristics, impacts and mechanisms of rapidly transitioning disparate precipitation states on the Canadian Prairies and will aid in better understanding both past and projected future hydro-climatic extremes in the region.

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Acknowledgments

This research was supported by the Canadian Foundation for Climate and Atmospheric Sciences and the Changing Cold Regions Network funded by the Natural Sciences and Engineering Research Council of Canada. The authors appreciate the assistance of Lucie Vincent, Éva Mekis and Ewa Milewska for providing the CANGRD temperature and precipitation data, respectively; Vincent Fortin and Bruce Davison for providing the CaPA data; and John Little of the Canadian Forest Service for providing the wildfire data and area-burned shapefiles. NCEP reanalysis data were provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, US, from their website at http://www.esrl.noaa.gov/psd/. The authors would also like to thank the two anonymous reviewers for their very helpful reviews.